Radio telephones are required by law to adhere to Electromagnetic Compatibility (EMC) limits as laid down in Type Approval specifications for Mobile Phones. EMC is defined as the ability of a device to function properly in its intended electromagnetic environment and to not be a source of electromagnetic pollution to that environment.
To enable electronic apparatus to be electromagnetically compatible, electromagnetic (EM) shielding is often placed within the apparatus to reduce the radiation emissions from radiating sources and to reduce the levels of radiation reaching sensitive components.
In known radio telephone designs interfering electronic circuits are segregated onto a number of circuit boards. The circuit boards are located in spaces within the frame structure of the telephone which are separated by high partitions to inhibit unwanted EM radiation passing between the circuits. The frame structure tends to be relatively large and generally forms part of the outer surface of the telephone.
In other known radio telephone designs the EM shielding of the electronic circuits is implemented using either metal casings or metallized plastic casings fixed onto a circuit board by soldering. A drawback of this design is that the inspection and repair of soldered electronic components is blocked by the presence of the EM shield casings. To overcome this drawback the EM shield casing is de-soldered, Inspection and repair performed on the soldered electronic components, then the casing is re-soldered onto the circuit board.
In the patent publication FI-85204 a frame structure of a radio phone is disclosed in which the frame comprises a substantially conducting frame plate, whereby the frame plate together with the ground foil of the circuit board forms the EMC shielding for the components located on the side of the circuit board facing the frame plate.
In the Finnish patent application FI-915242 a method is disclosed for producing a sealing compound to secure a EM shield case on to a circuit board thus preventing radio frequency (RF) radiation or conducting electricity on the case from disturbing other components in the telephone. The patent application also discloses a method for installing a circuit board to be RF shielded for example on the frame of a radio telephone.
In prior art designs, one of the problems has been the interference radiation emitted by the radio telephone and the EM shielding arranged for the prevention and attenuation of said radiation. The prior art designs are either too inefficient or too complex end difficult to implement. The prior art designs also include a number of circuit boards, which makes the entire assembly and the design of the EMC shielding more difficult.